The invention relates to a medium voltage electrical circuit breaker comprising a sealed enclosure filled with a high dielectric strength gas such as sulphur hexafluoride, a pair of main contacts located in said enclosure, a vacuum cartridge having a cylindrical housing sealed off by two end-plates. The vacuum cartridge is located in said enclosure and contains a pair of aligned arcing contacts which are electrically connected in parallel to said main contacts. The circuit breaker includes an operating mechanism to open the arcing contacts after the main contacts open and to close them before the main contacts close and a coil for producing an axial magnetic field in the formation zone of an arc, the magnetic field being drawn inside the cartridge when separation of the arcing contacts takes place.
A state-of-the-art circuit breaker (U.S. Pat. No. 5,155,315) of the kind mentioned, comprises an axial blowout coil incorporated in the cartridge. The coil is formed by notches arranged in the end-plate of the cartridge. This part is delicate to manufacture and only a fraction of the current flows along the spiral trajectory constituting the coil. The spectrum of the magnetic field generated by the coil is not ideal and the need has arisen to achieve a simplified device with improved performance.
It has already been proposed to shape the contact parts in such a way as to impose a current trajectory in the form of a spiral, to generate the magnetic field in the arcing zone. This solution has the above-mentioned drawbacks of complex parts and of an imperfect magnetic field spectrum.